1. Field of the Invention
The present invention is directed to a shock wave generator for use in an extracorporeal lithotripsy apparatus of the type wherein a shock wave is generated, and is propagated in a liquid-filled housing and is focused onto the calculi by a focusing element in the shock wave generator.
2. Description of the Prior Art
Shock wave generators are known in the art which generate a shock wave or pressure wave front, and wherein a plate-shaped element is disposed in the shock wave generator between the origin of the shock wave and the shock wave exit. The plate-shaped element has a smaller transverse area (i.e., the area in a plane perpendicular to the propagation direction of the shock wave) than the shock wave and consists of a material having an acoustic impedance which deviates from the acoustic impedance of the liquid. Such a shock wave generator is described, for example, in German Patent No. 32 40 691. Because the transverse area of the plate-shaped element is smaller than that of the shock wave, a portion of the shock wave can pass by the plate-shaped member unimpeded, whereas another portion of the shock wave passes through the material of the plate-shaped member. Because the acoustic impedance of the material comprising the plate-shaped member is different form the acoustic impedance of the surrounding liquid, that portion of the shock wave interacting with the material of the plate-shaped member is multiplied into a sequence of shock wave fronts due to multiple reflections at the front and rear sides of the plate-shaped member. The chronological spacing between the shock wave fronts is critically dependent on the thickness of the plate-shaped member. These multiple shock wave fronts are superimposed on that portion of the shock wave which passes the plate-shaped member unimpeded, so that a number of shock wave fronts act on the calculus. The mechanical stresses respectively produced by these fronts are superimposed on the calculus, so that an improved disintegrating effect, in comparison to a single shock wave front, is achieved.
In this known shock wave generator, a pressure curve of the type shown in the example of FIG. 1 with respect to time, (the time axis being disposed at a level corresponding to atmospheric pressure, or some other nominal pressure) occurs at the focus of the shock waves. This is composed of a theoretically infinitely large number of pressure pulses generated by multiple reflections which follow each other in constant chronological spacings, pressure pulses 2a through 2d in FIG. 1 being shown by way of example. The amplitudes of these subsequent pressure pulses decrease in a geometrical series. A pressure pulse 1, corresponding to the aforementioned portion of the shock wave which did not interact with the material of the plate-shaped member, is superimposed on the pressure pulses 2a through 2d. Depending upon whether the speed of sound propagation in the liquid is lower or greater than the propagation speed of sound in the plate-shaped member, the pressure pulse 1 may lag or lead the pressure pulse 2a. The individual pressure pulses each exhibit an extremely steep rise and a substantially exponentional decay, generally concluding in an undershoot 3, i.e., a considerable under-pressure briefly occurs under certain conditions. Such an undershoot can also exhibit the resultant chronological path of the pressure curve as a result of the addition of the pressure pulses. There are indications that the underpressure resulting from the drop in pressure in the region of the undershoot produces damage to the tissue surrounding the calculus which is to be destroyed. This damage is produced due to cavitation. Pressure curves which do not exhibit undershoots and are suitable for disintegrating calculi cannot be generated with the known shock wave generator as described above. Because of the multitude of pressure pulses which arise due to the multiple reflections, this known shock wave generator permits the chronological pressure curve at the focus to be modified only to a very limited degree. Another disadvantage of this known shock wave generator is that the multiple reflections at the boundary surfaces between the plate-shaped member and the liquid result in energy losses.